But i guarantee you on this you will be happy when you die. ...
No, you actually
don't. A guarantee carries compensation in case the thing that's guaranteed fails. For example, say your lawn mower is guaranteed not to break down for the period of a year. Typical compensation would include, say, the repair of that lawn mower, in cases where it does indeed break down in under a year.
You aren't offering any compensation, so your guarantee is totally empty and void of responsibility on your part. You aren't able to say to curiousgirl, "Oh, if you die and I was wrong, I'll give you a cookie!" - because curiousgirl would be, you know,
dead.
What you are offering is a
promise, not a guarantee. And a promise from you on this topic carries
no weight at all, because you havn't shown that you know that anything you're saying is true. Especially since you can't even speak from personal experience.